Direct numerical simulation of a pulverized coal jet flame employing a global volatile matter reaction scheme based on detailed reaction mechanism

Abstract

A two-step global reaction scheme for the volatile matter of coal is proposed, and the unsteady coal particle and combustion behaviors in a turbulent pulverized coal jet flame are investigated by performing a direct numerical simulation (DNS) employing the proposed global reaction scheme. The two-step global reaction scheme is constructed to take into account the properties of the volatile matter such as transport coefficients, laminar flame speed and unburned gas temperature and to be applicable to various coal types, and it is validated by comparing the results with those obtained by the detailed reaction mechanism which includes 158 chemical species and 1,804 reactions. The validity of the DNS is also assessed by comparing the results with those in the previous experiment (Hwang et al., 2005) [16], and the unsteady coal particle motions and combustion characteristics are examined in detail. The results show that the proposed two-step global reaction scheme for the volatile matter of coal can precisely predict the laminar flame speed and burned gas temperature for various coal types from bituminous to low-rank coals over wide ranges of conditions of equivalence ratios, pressures and unburned gas temperatures. In addition, it can correctly take into account the effects of dilutions by H2O and CO2 which compromise the evaporated moisture from coal and products of char reaction. It is also verified that a lab-scale turbulent pulverized coal jet flame is well predicted by the DNS employing the proposed global reaction scheme. That is, the pulverized coal particles’ velocity and its fluctuation and the characteristics of particle preferential motions are in general agreement with those observed in the experiment. The DNS reveals that in the turbulent pulverized coal jet flame, there appear premix and diffusion flame layers inside and outside, respectively. In addition, the reaction of the volatile matter and O2 in coal-carrier air occurs in the inner premixed flame layer, whereas the reactions of the volatile matter and CO and O2 in surrounding air occur in the outer diffusion flame layer.